1. Field of the Invention
The present invention relates to an image capturing apparatus and a method for controlling the same, and more particularly to an image capturing apparatus that has a function of detecting an object, and a method for controlling the same.
2. Description of the Related Art
An image capturing apparatus in which a photoelectric conversion element is used, such as a digital camera or a video camera, can detect an object from captured images using an image processing technique. Accordingly, image capturing apparatuses that perform auto focus control (AF) that causes an optical system of the apparatus to focus on a detected object and auto exposure control (AE) that sets exposure parameters to the apparatus such that the detected object is to be captured with an appropriate exposure are realized.
For example, a case is considered in which, in order to cause a display apparatus of an image capturing apparatus to function as an electronic viewfinder (EVF), continuous detection of an object is performed in EVF images (also referred to as live images, through-the-lens images, live view images, and so on) that are successively captured. In such a case, it is possible to employ, for example, a method disclosed by Japanese Patent Laid-Open No. 2007-274587 in which an object is detected from individual images using an image recognition technique such as face detection, or a method disclosed by Japanese Patent Laid-Open No. 2001-243478 in which a movement is detected (tracked) in a region designated as an object region.
With face detection, an object is detected from individual images by extracting a face feature, such as an eye or mouth, from the images and determining whether the object is a face. With the method of tracking an object region, on the other hand, an object is detected from individual images by searching a frame image for a region that has a high correlation with the object region that has been selected in another frame image. Hereinafter, for the sake of convenience, detection of an object using an image recognition technique such as face detection is referred to as “face detection”, and detection of an object by detecting a movement in the object region is referred to as “moving-subject detection” throughout the specification. It should be noted, however, that they are not intended to limit the object to a human face.
Because face detection requires the recognition of each of a plurality of parts constituting a face, the accuracy of object detection is high, but the computation is complicated and it takes time in processing, so face detection is inferior to moving-subject detection in terms of object detection speed (tracking capability). In contrast, with moving-subject detection, an object is detected through a differential operation between a plurality of frame images, so the processing speed is faster than face detection, but when an object's luminance changes, the detection becomes difficult, so moving-subject detection is inferior to face detection in terms of detection accuracy.
As described above, with the conventional method of tracking an object region, when priority is given to object detection accuracy, the detection speed decreases, and when priority is given to detection speed, the detection accuracy decreases.